The present invention relates to a semiconductor device and a lead frame for a semiconductor device. More specifically the present invention relates to a semiconductor device having high quality and reliability, which is obtained by fixing and holding a semiconductor chip whose one side is 70% of the length of one side of the outside dimension of a molded resin, onto a lead frame, and by sealing the semiconductor chip with the resin, and relates to a lead frame used for the semiconductor device.
In a conventional semiconductor device, a dimension of a die pad of a lead frame is 0.2 to 0.4 mm larger than a size of a semiconductor chip in consideration of mounting accuracy onto a die pad of the device at the time of die-bonding of the semiconductor chip to the die pad. When the die pad is made of Cu-based material, there is a great difference between the coefficient of thermal expansion of the semiconductor chip .alpha.si =3.5.times.10.sup.-6 and the coefficient of thermal expansion of Cu .alpha.cu=17.times.10.sup.-6. For this reason, when the semiconductor chip is die-bonded to the die pad, a difference in a thermal stress is generated, and thus the semiconductor chip is cracked in the horizontal direction and a molded for package is cambered more than 100 .mu.m due to heat distortion at the time of molding for package. Therefore, a residual thermal stress, such as soldering heat resistance, exists and the heat cycle characteristic is occasionally deteriorated. For this reason, the semiconductor chip having a dimension of up to 4 mm.times.4 mm has been normally mounted onto the die pad.
Accordingly, in order to solve the problem of thermal residual stress and to be able to mount a semiconductor chip having a dimension of not less than 4 mm.times.4 mm, the material for the die pad is changed to an Fe-based material having a coefficient of thermal expansion .alpha.fe of 5.5.times.10.sup.-6. However, the heat radiation characteristic of the semiconductor device is determined by a material for the die pad, and in the case of a Fe-based die pad, Fe thermal conductivity .gamma.fe is as small as 0.0159 W/mm.degree. C. compared to Cu thermal conductivity .gamma.cu of 0.0159 W/mm.degree. C. Therefore, a value of permissible electric power consumption of the finished semiconductor device becomes small.
Meanwhile, for example, Japanese Unexamined Patent Publication No. Hei. 7-202105 and Japanese Unexamined Patent Publication No. Hei. 8-236685 disclose examples of the die pad. However, in these publications, (1) the dimension of a semiconductor chip is smaller than an outer dimension of the die pad, (2) the dimension of the semiconductor chip is smaller than the outer dimension of reinforcement, (3) a small semiconductor chip whose dimension is 30% of dimension of the mold for package is stored in the mold, and the dimension of the semiconductor chip is equal to or smaller than a dimension from the outer size of the semiconductor chip to the outer size of the mold, (4) a gap between the die pad and the reinforcement is approximately equal to a difference between the dimensions of the semiconductor chip and die pad, and a width of the reinforcement is approximately equal to the gap between the die pad and reinforcement, (5) a material for the die pad is not described, (6) there is no description that a sunk amount of a Cu-based die pad should be larger than a sunk amount of a Fe-based die pad, and (7) there is no description about a relationship between rigidity of a die pad hanging lead and a dimension of the die pad. For this reason, phenomena which occur in an assembly procedure of the semiconductor device, such as a chip camber which occurs due to die-bonding, a change in a sunk amount of a frame due to the chip camber, a change in a sunk amount of the frame after wire bonding, and all phenomena which occur due to unbalance of resin fluidity at the time of molding, cannot be dealt with, so that a proportion of the dimension of the semiconductor chip stored in the mold cannot be made to be larger.
In future, when the function of the semiconductor chip is improved and the number of pins is increased, the volume of the semiconductor chip with respect to the volume of resin sealing becomes larger. At the same time, volumes of the die pad, die pad hanging leads and inner leads become larger.
For this reason, when the semiconductor chip is die-bonded to the die pad, the die pad is bent due to joining of materials with different coefficients of thermal expansion, and thus a dam bar and a lead frame are deformed via a hanging lead for supporting die pad. For example, when the rigidity of the hanging lead is closer to the rigidity of the lead frame, the lead frame is influenced by bending of the die pad, and thus the lead frame is deformed. For this reason, a dimension of a pitch of a transporting perforation provided in the frame in the assembly procedure of the semiconductor device is affected, and thus improper frame transportation into a wire-bonding device occurs.
On the contrary, when the rigidity of the hanging lead is very much smaller than the rigidity of the lead frame, the effect of the die pad bending on the lead frame is decreased, and thus the lead frame is not deformed. However, when a metal mold is removed after the lead is wire-bonded by pressing it using the metal mold in the wire bond procedure, there arises a problem that a lift-up amount of the semiconductor chip becomes larger due to a tension of a metallic wire. Moreover, in the case where the semiconductor chip is sealed by molten resin using a metal mold in the molding procedure, there arises a problem that when the molten resin is poured into the metal mold, an amount of shifting the die pad in the up-and-down direction becomes larger due to flowing pressure of the molten resin.
When a proportion of the volume of a semiconductor chip to the volume of resin sealing becomes larger, an effect of the aforementioned phenomena becomes remarkable, and in some cases, production of semiconductor devices becomes difficult. If such a semiconductor device is produced, the device is cambered greatly, and the device does not satisfy the standard of evenness of the lead. For this reason, there arise problems in that the yield is lowered, and resistance to solder and heat, resistance to heat cycling, resistance to cracking and resistance to release are not satisfactory.
In view of the above circumstances, it is an object of the present invention to provide a semiconductor device with high quality and reliability, and a lead frame used for the semiconductor device, in which (1) camber is reduced and die pad shifting is reduced at the time of die-bonding, (2) die pad shifting at the time of wire-bonding is reduced, and (3) die pad shifting at the time of molding is reduced, and thus the proportion of the volume of a semiconductor chip to the volume of sealing resin becomes larger.